This is the child Dn used as the lookup key in the inverse containment mapping table, the object whose parent an admin or tool wants to find. A manager indexes into this table by a known child Dn specifically to retrieve its parent efficiently, which is the reverse direction of the forward containment table's parent-to-child lookup. Supplying a child Dn like sys/chassis-1/fan-module-2 as the index here would return that fan module's containing chassis via the paired parent field.